A dynamic random access memory integrated circuit (DRAM) provides temporary storage of digital information. A distinctive feature of the DRAM is that the information stored in the circuit is quickly lost unless it is refreshed. The reason that information storage is only temporary in a DRAM is that the data storage is in the form of a charged capacitor. The cell shown in FIG. 1 is the heart of the memory circuit. It includes a word line 100 and bit line 102 connected to a pass transistor 104 and a capacitor 106. When the voltage on the word line 100 is raised, the pass transistor 104 turns on and the bit line 102 is connected to the storage capacitor 106. The information stored in the cell corresponds to whether the storage capacitor is charged or discharged. Unfortunately, capacitors leak charge and if not refreshed, the cell containing information corresponding to a charged capacitor would soon contain information corresponding to a discharged capacitor.
A natural solution to the problem of charge leakage is simply to increase the size of the capacitor. This approach, however, runs counter to the constant need for a smaller cell size, since in many DRAM circuits the storage capacitors alone can occupy as much as fifty to sixty percent of the die area of the circuit. The high premium placed on die area has resulted in cell designs in which the storage capacitor is formed not on the substrate surface, but instead on a protrusion that extends above the substrate surface. Such vertically-formed capacitors are known in the industry as a “stacked cell.” The use of a stacked cell allows for a higher storage capacitance without occupying precious semiconductor die area.
FIG. 2a shows a prior art planar DRAM cell and FIG. 2b shows a prior art stacked cell. In FIG. 2a the “storage node,” or the terminal of the capacitor connected to the transistor lies in the semiconductor substrate 200 beneath the capacitor dielectric 202. The other terminal, or field plate, of the planar capacitor is typically polysilicon and is shown as element 204. The wordline 206 comprises the gate interconnection for the pass transistor and lies over the gate dielectric 208 and between the source and drain implantation regions 210. The bitline 212 runs perpendicularly to and over the wordline and storage capacitor. In the stacked cell shown in FIG. 2b, both plates of the capacitor are polysilicon. The storage node 250 is convoluted and only contacts the substrate at transistor contact region 260. The wordline 256 and bitline 262 are in essentially the same position as in the structure of FIG. 2a. The capacitor dielectric 252 is typically oxide or a combination of oxide and nitride. The field plate 254 conforms to the convolutions of the storage node 250 to create a capacitor with a larger surface area than with the capacitor of FIG. 2a. The drastically reduced die area occupied by the stacked cell of FIG. 2b is also apparent in a comparison with the structure of FIG. 2a. 
Designers of future generations of DRAMs demand that the storage capacitor occupy even less die area than that of the structure shown in FIG. 2b. One problem with conventional processes for forming stacked cell capacitors is that the vertical nature of the capacitor requires relatively thick layers (typically oxide) for the capacitor's formation. Contact from the capacitor to the contact region (source or drain) of the pass transistor is complicated by the thick layers since the area of contact is often less than 0.5 μm in dimension, and will continue to be made smaller in future DRAM generations. Forming such a small opening in thick layers is very difficult and is a source of process complexity. For example, conventional processes rely on polysilicon or silicon nitride hardmasks with openings of about 0.36 μm to gain the selectivity necessary for etching such small holes in thick layers of oxide. Even using such a hardmask, the etch depth that can be achieved is often only about 1.0 μm when a depth of approximately 50% or greater is desired. To achieve the needed depth, prior processes typically rely on multiple masking steps where the bottom portion of the capacitor contact to the transistor is formed before the thick oxide layers are applied. The hole is then plugged with polysilicon and the thick layers necessary for forming the upper portions of the capacitor follow. The multiple mask steps necessary to form the capacitor in prior processes are thus complicated and economically unattractive. The present invention provides a simpler approach to the formation of stacked capacitors.